1. Field of the Invention
This invention relates to a vibration-coupling type ultrasonic actuator capable of being effectively activated by coupling multi-mode vibrations excited with single phase current, and a method capable of coupling multi-mode vibrations excited with the single phase current by controlling the electrical and vibrational characteristic of a piezoelectric member so as to operate the ultrasonic actuator with high efficiency.
2. Description of the Prior Art
When a plate-like elastic vibrational member is excited by frequencies resonant with one or more factors of the length, width and thickness of the vibrational member, standing-wave vibrations (elastic wave) of some inherent modes can be observed. There are caused vibrations of longitudinal mode, transverse (thikness) mode, bending (flexural) mode, torsional mode, sliding mode, spreading mode and other possible modes. Recently, these modes of vibrations have been widely applied to mechanical-electric filters, various ultrasonic motors and so on. Since the ultrasonic motor has no need of a moving member such as a rotor used in a common electric motor and can serve as a linear motor applicable to, for example, peripheral equipments for business computers such as card readers, and sheet-feeding machines in copying machines. The motor of this type can activate directly a moving object such as paper in the card reader with efficiency.
The vibrations of the foregoing modes all are the so-called standing-wave vibrations having periodic motions such that the elastic vibrational member expands and contracts repeatedly in one direction. As illustrated in FIG. 1, the longitudinal mode stands for a stretching vibration caused in the lengthwise direction of the elastic vibrational member (L-mode vibration). In the bending mode, the elastic vibrational member is caused to be periodically strained in the opposite directions at the upper and lower parts differentiated by the neutral plane Pn (B-mode vibration). In one of the vibration modes, a stretching motion caused inside the vibrational member in the orthogonal direction or specific direction is imparted to the moving member (rotor in a motor) which comes in point or face contact with the vibrational member. Consequently, the moving member being in contact with the vibrational member is subjected to a unidirectional rectilinear motion or rotary motion.
The ultrasonic motor merely utilizing a simple unidireclional-stretching motion caused on the elastic vibrational member is a so-called "poking-type ultrasonic motor" actuated by single-mode vibration (1st resonant in-plane vibration). Japanese Patent Publication NO. SHO 58(1983)-32518(B2) discloses a single-mode ultrasonic motor actuated by longitudinal mode vibration. This prior art motor has an advantage that it can be exited with single phase current. However, the motor of this type renders the controlling of alteration in direction of rotation and stable activation with strong torque difficult, and besides, cannot efficiently be actuated in the reverse direction even if it can be reversed.
It would be possible to excite a plurality of modes of in-plane vibrations (multi-mode vibrations), for example, the longitudinal vibration together with the bending vibration by applying polyphase currents to the elastic vibrational member.
There have been briskly conducted researches in utilization of the multi-mode (two-mode) vibrations for a driving system wherein two of the vibration modes including the longitudinal vibration, transverse vibration and bending vibration are typically combined. In a general sense, the multi-modes is comprehensive of the in-plane vibrations of the same mode which are excited with alternating currents differring in phase.
However, the ultrasonic motor using the multi-modes of vibrations entails problems such as difficulty in controlling the application of the polyphase alternating currents differring in resonance phase and complexity in structure.
In a case of exciting the elastic vibrational member with the polyphase currents, kinetic energy designated by the composite vectors of geometrically orthogonal displacements at arbitrary points on the surface of the vibrational member can be obtained in the form of an elliptic motion. In a vibrational system, where two modes of vibrations are "coupled" with resonant frequencies, the amplitude energy of one of the multi-mode vibrations partially enters into the other mode vibration, to thereby intensify the amplitude motion caused elliptically (elliptic motion) of the latter vibration.
As shown schematically in FIG. 1, the elliptic motion Ve is generated on the surface of an elastic vibrational member 1 shaped in a rectangular plate by applying two-phase alternating currents V1 (cos .omega.t) and V2 (sin .omega.t) having specific frequency to piezoelectric vibrators 3a, 3b attached to the vibrational member 1. The vibrational member 1 has the dimensions (length, width and thickness) designed so as to cause resonance with the alternating currents applied to the vibrators to thereby excite longitudinal (L-mode) vibration and bending (B-mode) vibration. The longitudinal and bending vibrations can concurrently be produced inside the elastic vibrational member 1 with either of the alternating currents, whereas the vibrations excited by the single-phase current can in no way be coupled. Therefore, the kinetic energy resultantly produced from the vibrations corresponds merely to the composite vector of the longitudinal and bending vibrations. The concurrent application of the polyphase alternating currents brings about the "coupling" of the longitudinal and bending vibrations to produce intensive elliptic motions (motion Ve at an arbitrary point P on the vibrational member 1). Reference symbol N in the drawing denotes the mode of the longitudinal vibration which is induced inside the vibrational member but does not vary in the transverse direction.
When the vibrational member is formed asymmetrically with respect to the neutral plane Pn, the longitudinal and bending vibrations are of course coupled with the single phase current, whereas it is impossible to form a vibrational member having the ideal geometric shape which effectively produces resonance in vibration from a technical viewpoint. The vibrational member which is awkward in resonance cannot efficiently be activated.
As is understood from the foregoing, there has been so far required polyphase currents having phase difference in order to obtain an ultrasonic motor excelling in driving torque, performance and controllability. Namely, the conventional ultrasonic motor has attained the coupling of the multi-mode vibrations to produce the elliptic motions on the elastic vibrational member by use of the polyphase alternating currents. Furthermore, in order to reverse the direction of the elliptic motions thus produced, the polarity of the power source to be applied to the vibrators has generally to be reversed. Employing the method of reversing the polarity of the power source, disadvantageously, the structure of a controlling system for the ultrasonic motor would be complicated.
Even if the elastic vibrational member on which the elliptic motions are induced may be so formed as to be resonated with the frequency of the exciting vibrations, the resonant frequency of the vibrational member undergoes a change due to physical factors such as the mass changes which are caused by providing the piezoelectric vibrators and electrodes onto the vibrational member. As a result, the multi-mode vibrations would fail to be coupled, and therefore, the ultrasonic motor could not be activated with high efficiency.